From f3775dc2df0e927aa99c852fd2d8b613a33f91b0 Mon Sep 17 00:00:00 2001
From: Johannes Stoelp <johannes.stoelp@gmail.com>
Date: Tue, 7 Nov 2023 21:51:26 +0100
Subject: mutex: add owning mutex utility

---
 Makefile             |   1 +
 owning_mutex.h       | 125 +++++++++++++++++++++++++++++++++++++++++++++++++++
 test/owning_mutex.cc |  41 +++++++++++++++++
 3 files changed, 167 insertions(+)
 create mode 100644 owning_mutex.h
 create mode 100644 test/owning_mutex.cc

diff --git a/Makefile b/Makefile
index 8d1fb21..e489d12 100644
--- a/Makefile
+++ b/Makefile
@@ -2,6 +2,7 @@ TEST += bitfield
 TEST += option
 TEST += timer
 TEST += log
+TEST += owning_mutex
 
 # -- INTERNALS -----------------------------------------------------------------
 
diff --git a/owning_mutex.h b/owning_mutex.h
new file mode 100644
index 0000000..82756da
--- /dev/null
+++ b/owning_mutex.h
@@ -0,0 +1,125 @@
+#ifndef UTILS_MUTEX_H
+#define UTILS_MUTEX_H
+
+#include <mutex>
+
+#include <cassert>
+
+template <typename T, typename M>
+struct guard;
+
+// -- OWNING MUTEX -------------------------------------------------------------
+
+/// owning_mutex
+///
+/// An mutex wrapper type that owns a value of type T and provides mutual
+/// exclusive access to that value through guard objects. Guard objects are
+/// obtained from the wrapper API. When a guard goes out of scope, the mutex
+/// will be unlocked automatically.
+///
+/// The mutex type can be controlled by the template type argument M.
+///   M: BasicLockable
+///
+/// EXAMPLE:
+///   struct data { int a; };
+///   owning_mutex<data> val{1};
+///
+///   {
+///     auto guard = val.lock();
+///     guard->a = 1337;
+///     // mutex will be unlocked after this scope
+///   }
+template <typename T, typename M = std::mutex>
+struct owning_mutex {
+  template <typename... Args>
+  constexpr explicit owning_mutex(Args... args)
+      : m_val{std::forward<Args>(args)...} {}
+
+  owning_mutex(const owning_mutex&) = delete;
+  owning_mutex(owning_mutex&&) = delete;
+
+  guard<T, M> lock() {
+    return {m_mtx, m_val};
+  }
+
+ private:
+  M m_mtx;
+  T m_val;
+};
+
+// -- GUARD --------------------------------------------------------------------
+
+#if __cplusplus >= 201703L
+
+template <typename T, typename M>
+struct [[nodiscard]] guard {
+  guard(M& mtx, T& val) : m_lk{mtx}, m_val{val} {}
+
+  // With the guaranteed copy elision (cpp17) we can truly delete the
+  // copy/move constructor of the guard type.
+  //
+  // https://stackoverflow.com/a/38043447
+  guard(const guard&) = delete;
+  guard(guard&&) = delete;
+
+  T& operator*() {
+    return m_val;
+  }
+
+  T* operator->() {
+    return &m_val;
+  }
+
+ private:
+  std::lock_guard<M> m_lk;
+  T& m_val;
+};
+
+#else  // before cpp17
+
+template <typename T, typename M>
+struct guard {
+  guard(M& mtx, T& val) : m_mtx{&mtx}, m_val{val} {
+    m_mtx->lock();
+  }
+
+  ~guard() {
+    if (m_mtx) {
+      m_mtx->unlock();
+    }
+  }
+
+  T& operator*() {
+    assert(m_mtx != nullptr);
+    return m_val;
+  }
+
+  T* operator->() {
+    if (!m_mtx) {
+      return nullptr;
+    }
+    return &m_val;
+  }
+
+  guard(const guard&) = delete;
+  // Implement move constructor for cases where the compiler does no copy
+  // elision.
+  // For API compatibility with the cpp17 version, the move constructor
+  // should not be explicitly invoked by the user.
+  //
+  // SAFETY: Exclusive access to T is guaranteed as at any given time only a
+  // single *guard* instance is NOT moved.
+  //
+  // UB: The *guard* must not be moved across thread boundaries and dropped
+  // there.
+  guard(guard&& rhs) noexcept : m_mtx{rhs.m_mtx}, m_val{rhs.m_val} {
+    rhs.m_mtx = nullptr;
+  }
+
+ private:
+  M* m_mtx;
+  T& m_val;
+};
+#endif
+
+#endif
diff --git a/test/owning_mutex.cc b/test/owning_mutex.cc
new file mode 100644
index 0000000..060415f
--- /dev/null
+++ b/test/owning_mutex.cc
@@ -0,0 +1,41 @@
+#include <owning_mutex.h>
+
+#include <limits>
+#include <thread>
+#include <vector>
+
+#include <cassert>
+#include <cstdio>
+
+constexpr unsigned kNumThreads = 8;
+constexpr unsigned kIter = 1 << 18;
+
+static_assert((static_cast<unsigned long>(kNumThreads) *
+               static_cast<unsigned long>(kIter)) <=
+                  std::numeric_limits<unsigned>::max(),
+              "Expectate result overflowed!");
+
+int main() {
+  owning_mutex<unsigned> data(0u);
+
+  std::vector<std::thread> threads;
+  threads.reserve(kNumThreads);
+
+  for (unsigned t = 0; t < kNumThreads; ++t) {
+    threads.emplace_back([&data, t]() {
+      for (unsigned i = 0; i < kIter; ++i) {
+        *data.lock() += 1;
+      }
+      std::printf("th%u finished\n", t);
+    });
+  }
+
+  for (auto& th : threads) {
+    th.join();
+  }
+
+  assert(*data.lock() == (kNumThreads * kIter));
+  std::printf("Result %u\n", *data.lock());
+
+  return 0;
+}
-- 
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